The present invention relates to optical devices and their method of assembly.
Substrates used to construct optical devices must have a low density of imperfections which could block light transmission. When substrates are joined together, care must be taken to avoid surface imperfections that would scatter the light or cause irregularities such that the light transmission would be impeded.
In one method known in the art, substrates are carefully lapped to a flat surface. They are then heated to their softening temperatures, fused together, and cooled to ambient temperature. Because of the small difference in the coefficients of thermal expansion of the substrates used to make the optical devices and the large temperature difference between the fusing temperature and ambient temperature, thermal strains arise at the interface of the substrate upon cooling. As a result, a rippled surface pattern forms at the interface. This rippling effect causes excessive attenuation of light, reducing the efficiency of the optical device.
Adhesives which are conventionally used in bonding optical disk substrates include solvent-type adhesives, heat activatable adhesives, and two-component condensation-type cold curable silicone rubber-based adhesives. Solvent-type adhesives erode both the substrates themselves and the reflective films. They are also unsuitable for mass production, because the substrates have to be held stationary under uniform pressure for a long period of time until the adhesives are completely cured.
Heat activatable adhesives have the advantage of solidifying in a short period of time after activation. However, the heat applied during activation deforms the substrate. In optical disks, this prevents uniform rotation and, as a result, inexact reproduction of stored information.
Liquid adhesives produce irregularities which cause the bonded layers to deviate from optimum thickness. In optical disks, unless the thickness of the layers is maintained, the flatness of the signal recording layer is deteriorated. This makes it difficult to write and/or read data signals. When a substrate is bonded to another substrate with a liquid adhesive, it is difficult to avoid exudation. In other words, excess adhesive material is displaced and collects on the exterior of the composite article. The exudate can interfere with optical transmission or the packaging of the composite.
Numerous products are made which require the attachment of an optically transmissive substrate to another substrate material such as plastic or glass. Such products include optical memory devices and optical isolators. Optical memory products include optical disks, laser disks, and optical memories. Video disks or compact disks are designed for read-only capability. Information is retrieved by laser light. Optical memories are capable of both information recording and playback or erasure.
The latest version of the optical disk, a digital versatile disk has two layers of data pits. An upper surface is coated with a partially reflective, partially transmissive layer. The reflectivity of the upper layer is sufficient to enable a laser beam to read the pits in the upper substrate. The transmissivity of the upper layer allows the laser beam to focus on and read pits in the second layer. Accordingly, these disks can store more information than conventional disks. In order to function properly, an adhesive of superior quality is needed to bond the two substrates together. The thickness of the bond must be uniform to prevent distortion of focused readout spots.
Thus, there exists a need for a reliable and efficient method of bonding substrates together for use in critical optical applications such as optical memory products and optical isolators. The present invention is directed toward fulfilling this need.
The present invention relates to a method of forming an optically transmissive substrate by providing a pressure sensitive adhesive sheet which is optically transmissive and has release liners on both sides, removing a release liner from one side of the pressure sensitive adhesive sheet, and applying the exposed side of the pressure sensitive adhesive sheet to one side of the optically transmissive substrate.
Another aspect of the present invention is the optically transmissive substrate with a preapplied adhesive layer having an optically transmissive substrate, a release layer, and a pressure sensitive adhesive sheet which is optically transmissive and is secured to and positioned between the optically transmissive substrate and the release liner.
A further embodiment of the present invention is an optical element having an optically transmissive substrate, a compatible substrate, and a pressure sensitive adhesive sheet which is optically transmissive and is secured to and positioned between the optically transmissive substrate and the compatible substrate.
Yet another embodiment of the present invention is a method of forming an optical isolator by providing a Faraday rotator and first and second pressure sensitive adhesive sheets which are optically transmissive and have release liners on opposed sides of the sheets. A first release liner is removed from a first side of each of the pressure sensitive adhesive sheets. The first side of the first pressure sensitive adhesive sheet is applied to a side of a first polarizer. The first side of the second pressure sensitive adhesive sheet is applied to a side of a second polarizer. The second release liner is removed from each of the adhesive sheets. The second side of the first adhesive sheet is applied to a first end of the Faraday rotator, while the second side of the second adhesive sheet is applied to a second end of the Faraday rotator.
Another aspect of the present invention is an optical isolator having a Faraday rotator, two polarizers, and two adhesive sheets each which is bound on one side to one of the polarizers and on the other side to one end of the Faraday rotator.